This invention is concerned with a method of drying a coating of solvent-based adhesive applied to a surface of an article, e.g. a shoe.
In the drying of a layer of solvent-based adhesive applied to an article surface, the total drying time can be divided into two separate periods:
In a first period the rate of evaporation of solvent is determined by the state of the air immediately adjacent the surface, this rate being constant while the surface remains "wet". Thus during this period, which is referred to as the "constant rate period", the rate of drying depends upon the rate of removal of solvent vapour from the layer of air adjacent to the wet surface.
In the second period, the rate of removal of solvent depends upon the migration of the solvent through the moist solid to the surface, the resistance to solvent transfer progressively increasing and thus reducing the drying rate. Thus during this period, which is referred to as the "falling rate period", the controlling factor is the rate of vapour diffusion through the solid.
While it is of course possible to allow the solvent to evaporate off naturally, it is usual for force-drying to take place in order to accelerate the rate of evaporation/drying. To this end, the surface to which the adhesive has been applied may be heated, e.g. by radiant heaters; alternatively, the surface may be subjected to hot air which in passing over the article surface serves also to remove solvent-laden air from adjacent to it. In any event, it is generally considered that heat is desirable in both the constant rate period and the subsequent falling rate period, the amount of heat supplied being dependent upon the amount of time which can be allocated to the drying process.
It will of course be appreciated that use of heat in this way is expensive in sofar as either the energy costs are high if the air is not re-circulated, or, if the air to be re-circulated, then a more complicated apparatus is required, usually also involving thermal insulation panels, as well as a system for removing solvent from the now solvent-laden air.